This invention relates in general to time base generator circuits, and particularly to circuits which produce digital timing signals used for calibrating time measurement systems.
Digital time base generator circuits are circuits which produce a signal having pulse edges separated by a known relationship in time, hereinafter called a time reference signal. Time base generator circuits are also known as timing signal generators or time standards. To calibrate a time measurement system precisely, a time base generator circuit should produce a signal having a period which is the same length or smaller than the minimum time period to be measured. Circuits which produce a signal in the range of one Hz, for example, are commonly used in watches and clocks, where one second is the minimum time period to be measured.
The manufacture and use of electronic circuits requires that parameters such as switching speeds and gate delay times are measured accurately. Commonly, time measurement circuits are used to measure these parameters. Time measurement circuits produce an output that is proportional to the amount of time that passes between two events, usually between two pulses on a signal line. Calibration of these time measurement systems involves inputting a signal with a known time delay and comparing the measurement systems output with the known time delay. Usually the input signal comprises a start signal and stop signal wherein an edge of the start signal triggers the time measurement system to begin measuring time and an edge of the stop signal triggers the time measurement system to stop. The separation, or delay, between the edges of the start and stop signal must be extremely accurate in order to calibrate the time measurement circuits. Because switching speed and gate delay time of some circuits is in the order of picoseconds (ps), it is necessary for the time measurement system to be precise in the range of just a few picoseconds.
In the past, circuits which produced a timing signal, or a pair of timing signals to be used as the start and stop signal described above, comprised an oscillator which generated a reference signal which was then split between a first and a second transmission line. The first transmission line was coupled directly to the time measurement system and provided the start signal. The second transmission line comprised a mechanical delay line of a known time delay .DELTA.t. A signal traveling through the second transmission line, therefore, took longer to reach the end of the second transmission line than did the signal traveling through the first transmission line. Thus, in theory, the signals on the first and second transmission lines would be identical except out of phase by a known time delay .DELTA.t which was determined by the length of the mechanical delay time. The signal on the first transmission line could be used as a start signal and the signal on the second transmission line could be used as a stop signal.
Circuits using mechanical delay lines are quite bulky since even a short delay requires several feet of transmission line. Such circuits are not compatible with portable equipment, and cannot be built into a piece of equipment without increasing the size and cost of the equipment. Thus, time base generators were usually external to a piece of equipment, such as a tester, and were used only occasionally to calibrate the equipment. Time base circuits which were small enough to be built into equipment lacked the precision for many applications.
The time base generator circuit described above resulted in standing waves on the first and second transmission lines caused by reflected energy which occurs at the termination of the transmission lines. The amplitude of the standing wave was a function of the frequency of oscillation on the transmission line and the characteristics of the transmission line termination. The standing wave interfered constructively or destructively with the signal on the transmission lines depending on the frequency of the standing wave and the length of the transmission lines. These effects became more pronounced as higher frequencies were transmitted on the transmission lines. Thus, the output received from the first and second transmission lines was dependent on the reference frequency and the length of the transmission lines.
To be useful for calibration purposes, a time reference circuit must be adjustable over some range of time periods so that various time reference signals can be applied to the time measurement system during calibration. In order to vary the time reference signal of the circuits described hereinbefore, two methods were commonly used. First, the oscillation frequency could be varied to change the period of the reference signal as well as the time delay between start and stop edges. Unfortunately, however, the standing waves were generated on the first and second transmission lines even when the transmission lines were properly terminated, and resulted in noise on the transmission lines which reduced the integrity of the time base signal. Because the first and second transmission lines were different lengths, the standing wave noise effected the start and stop signals differently. Because of this, the noise modified the time reference signal and appeared to the time measurement system as an increase or decrease in time delay of the time reference signal. Thus, to be truly accurate, the time reference circuit would itself have to be calibrated very carefully at each oscillator frequency before being used to calibrate a time measurement system. Since the amplitude of the standing wave increased at higher frequencies, the noise problem was particularly acute when time reference signals less than a few nanoseconds were needed.
Another method of using the above described time reference circuit is to use an adjustable mechanical delay line so that variable time delays can be generated. Variable mechanical delay lines are merely transmission lines wherein the length of the transmission line can be changed by manually expanding or contracting the delay line. Thus, the length of the second transmission line can be increased or decreased to change the relative position of the start and stop edges produced by the time reference circuit. Although this arrangement allowed the use of a single oscillator frequency which eliminated standing wave variation due to oscillator frequency, changing the length of the transmission line added a new noise component to the time base generator. The amplitude of a standing wave varies along the length of a transmission line so that as the mechanical delay line was increased in size the effect of the standing wave on the output time reference signal changed. This change was seen by the time measurement system as a change in reference time.
It should be understood that while the time base circuit described hereinbefore is adequate for relatively long time periods, it becomes difficult to use for sub-nanosecond time periods.
Accordingly, it is an object of the present invention to provide a time reference circuit which can provide various time references without changing reference frequency on the signal line.
It is another object of the present invention to provide a time reference circuit without a mechanical delay line.
It is a further object of the present invention to provide a time reference circuit which is compact and can be easily incorporated into a piece of equipment.
It is a further object of the present invention to provide a time reference circuit with improved precision.
It is still another object of the present invention to provide a time reference circuit which is easily programmable.